The present invention generally pertains to frequency dividers and is particularly directed to portable, batteryless, frequency dividers of the type that are included in tags that are used in presence detection systems.
Portable, batteryless, frequency dividers are described in U.S. Pat. No. 4,481,428 to Lincoln H. Charlot, Jr., U.S. Pat. No. 4,670,740 to Fred Wade Herman and Lincoln H. Charlot, Jr., U.S. Pat. No. 5,065,137 to Fred Wade Herman and U.S. Pat. No. 5,065,138 to Ming Lian and Fred Wade Herman.
The frequency divider described in the '428 patent includes a resonant first circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency, and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency; and the two resonant circuits are electrically connected to one another by a semiconductor switching device having gain coupling the first and second resonant circuits for causing the second circuit to transmit electromagnetic radiation at the second frequency solely in response to unrectified energy at the first frequency provided in the first circuit upon receipt of electromagnetic radiation at the first frequency. Each resonant circuit includes a fixed capacitance connected in parallel with an inductance coil. In order to minimize difficulties due to magnetic coupling between the coils when tuning the resonant circuits to their respective resonant frequencies the coils are disposed in relation to each other so as to avoid mutual coupling. Mutual coupling is defined in the '428 patent as coupling to such an extent as to decrease the efficiency of the frequency divider. Preferably the coils are disposed perpendicular to each other so that the magnetic fields of the two coils are orthogonal to each other.
The frequency divider described in the '740 patent consists of a single resonant circuit consisting of an inductor and a variable-capacitance diode (varactor) connected in parallel to define a resonant circuit that detects electromagnetic radiation at a first predetermined frequency and responds to said detection by transmitting electromagnetic radiation at a second frequency that is one-half the first frequency, wherein the circuit is resonant at the second frequency when the voltage across the diode is zero.
The frequency divider described in the '137 and '138 patents includes a first resonant circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency; wherein the first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit at the first frequency in response to receipt by the first circuit of electromagnetic radiation at the first frequency; and wherein the first resonant circuit and/or the second resonant circuit includes a variable reactance element in which the reactance varies with variations in energy received by and/or transferred from the first resonant circuit for causing the second resonant circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first resonant circuit at the first frequency.